1. Field of the Invention
The present invention relates to a method of manufacturing a thin quartz crystal wafer from a crystal body of synthetic quartz crystal, and more particularly to a method of manufacturing a thin quartz crystal wafer using a laser beam.
2. Description of the Related Art
Synthetic quartz crystal that is produced by growing quartz crystal according to hydrothermal synthesis or the like is known as a major material of electronic components typified by quartz crystal units. A quartz crystal unit comprising a quartz crystal blank cut from synthetic quartz crystal and hermetically sealed in a casing is used as a frequency control element in an oscillator or a filter. An AT-cut quartz crystal blank whose resonant frequency is inversely proportional to its thickness is widely used in such a crystal unit. A crystal blank is generally manufactured by cutting a thin quartz crystal wafer having a desired thickness. In recent years, as the communication frequency is as high as 100 MHz or higher, for example, a crystal blank used as a quartz unit has a thickness of about 18 μm or less. Efforts have been made to develop a process of manufacturing such a crystal blank.
FIGS. 1A to 1C show successive steps of a conventional process of manufacturing a thin quartz crystal wafer. Thin quartz crystal wafer 1 is cut from quartz crystal block 2 in the form of a rectangular parallelepiped having flat surfaces. As shown in FIGS. 1A to 1C, if an AT-cut crystal blank is to be finally cut out, then quartz crystal block 2 is cut from a crystal block of synthetic quartz crystal along predetermined orientations (X-, Y′-, and Z′-axes) of quartz crystal. The X-, Y′-, and Z′-axes refer to crystalline axes that are crystallographically determined for quartz crystal. Quartz crystal block 2 is cut by a wire saw or a blade saw along line A—A in FIG. 1A to produce relatively thick quartz crystal wafer 3 having a thickness along the Y′-axis. The thickness of thick quartz crystal wafer 3 is of about 350 μm. Thereafter, thick quartz crystal wafer 3 is polished or ground into thin quartz crystal wafer 1 having a prescribed thickness. If a crystal blank for use in a 100 MHz crystal unit is to be produced from thin quartz crystal wafer 1, thin quartz crystal wafer 1 has a thickness of about 18 μm. Then, thin quartz crystal wafer 1 is cut into individual crystal blanks along line B—B and line C—C in FIG. 1C by photolithographic etching.
Finally, as shown in FIG. 2, exciting electrodes 5 and extension electrodes 6 are formed on respective principal surfaces of crystal blank 4, extension electrodes 6 extending from respective exciting electrodes 5 to an end of crystal blank 4 and having portions folded back onto the other principal surfaces across the end of crystal blank 4. Crystal blank 4 with exciting electrodes 5 and extension electrodes 6 mounted thereon is hermetically sealed in a casing, and predetermined electric connections are made to extension electrodes 6, thus completing a crystal unit.
According to the above manufacturing process, however, thin quartz crystal wafer 1 is obtained from a thick quartz crystal wafer having a thickness of several hundreds μm by polishing or grinding in the unit of μm. Therefore, the manufacturing process produces material wastes and is low in productivity. Since a wafer cut by the machining process using a wire saw or a blade saw has a thickness ranging from 200 to 400 μm as a lower limit, it is necessary to polish or grind thick quartz crystal wafer 3 in order to produce thin quartz crystal wafer 1 therefrom.
A technique known as “stealth dicing” has been proposed for producing a thin silicon semiconductor wafer having a thickness of about 30 μm without polishing or grinding. This technique employs a laser beam having a wavelength that is transmissive with respect to a semiconductor wafer to be processed thereby. The laser beam is converged inside the semiconductor wafer to cause multiphoton absorption in the converged area, thereby forming an internally modified region from which the semiconductor wafer starts to be divided. Details of stealth dicing are disclosed in Takaoka Hidetsugu, “Principles and features of stealth dicing technique optimum for dicing ultrathin semiconductor wafers”, Electronic materials (Denshi Zairyou in Japanese) (ISSN 0387-0774), Vol. 41, No. 9, pp. 17–21, September 2002, and Japanese laid-open patent publication No. 2002-205181 (JP, P2002-205181A).